The present disclosure relates to a developing device configured to develop an electrostatic latent image using toner and relates to an image forming apparatus including the developing device.
Some image forming apparatuses, including multifunction peripherals, copiers, printers, and facsimile machines, perform printing by using toner to develop an electrostatic latent image formed on a photosensitive drum. In some of these image forming apparatuses, a developer (so-called two-component developer) containing a magnetic carrier and a toner is used. However, if, in the case of development using a two-component developer, a magnetic brush using the magnetic carrier is brought into direct contact with the photosensitive drum, various undesirable effects, such as poor image quality, will be produced. For this reason, there is proposed an image forming apparatus including a developing device of a development system (referred to also as a “touchdown development” or a “hybrid development”) in which an electrostatic latent image is developed without contact of a magnetic brush with a photosensitive drum by placing a developing roller for carrying toner to face the photosensitive drum, forming the magnetic brush on a magnetic roller placed to face the developing roller, and transferring only the toner to the developing roller using the magnetic brush. This development system is more advantageous than the one-component development system and the conventional two-component development system in various aspects, such as image quality, print speed, toner life, and anti-scattering of carrier.
There is also proposed an image forming apparatus configured to form an image on a paper sheet by using a developing roller capable of formation of a thin toner layer on the surface thereof and a magnetic roller capable of supplying toner to the developing roller by means of a magnetic carrier and developing an electrostatic latent image with the developing roller, wherein, without provision of any additional special member, a high-speed, small-sized, hybrid developing device advantageous in various aspects, such as image quality, toner life, anti-scattering of carrier, and print speed, is achieved.
In the above touchdown development system, an AC voltage (for example, about 1 to 2 kV from peak to peak) is applied to the developing roller to cause charged toner to fly toward and deposit on the photosensitive drum and thereby develop an electrostatic latent image. In some developing devices of this system, a signal representing ON/OFF of energization is generated by switching, a signal obtained by removing a DC component from the above ON/OFF signal using a capacitor is input to the primary side of a transformer, and an AC voltage to be applied to the developing roller is acquired from the secondary side of the transformer.
In this case, from the viewpoint of prevention of leakage (discharge) between the photosensitive drum and the developing roller and prevention of production of an uneven toner image, it may be desired to change the duty ratio for the switching. However, a change of the duty ratio for the switching results in the application of an unbalanced voltage (energy-biased voltage) to the transformer and so on. Therefore, the transformer may produce magnetic bias or magnetic saturation. If magnetic bias or magnetic saturation is produced to bias the magnetic flux, the transformer will be biased by a direct current, so that a large current (an eddy current) will be very likely to flow. In addition, if a large current over the rated current flows through the switching element, the switching element will be broken down. Furthermore, as the amount of duty ratio instantaneously changed is larger, a larger magnetic bias will be produced and a large current will be more likely to flow through the switching element.
To cope with this, it can be considered to change the duty ratio stepwise multiple times until a desired duty ratio is reached. In this case, whenever the duty ratio is changed, energy resonance (oscillation) between the transformer and the capacitor occurs to produce periodic fluctuation in voltage between the electrodes of the capacitor (and also on the transformer side thereof). In relation to this energy oscillation, the period of the fluctuation in voltage between the electrodes of the capacitor may include a timing (time slot) in which a large current is particularly likely to flow through the switching element. Therefore, a problem arises in that even if the duty ratio is changed stepwise, it is necessary to avoid a large current flowing though the switching element.
Furthermore, the period of fluctuation in voltage between the electrodes of the capacitor has a relationship with the period (or frequency) of resonance between the capacitor and the transformer. The resonance period (or resonance frequency) depends upon the capacitance of the capacitor and the inductance value of the transformer (the primary coil thereof). The capacitance and the inductance value depend upon temperature. Therefore, the time slot in which a large current is likely to flow through the switching element during the period of fluctuation in voltage between the electrodes of the capacitor also depends upon temperature. Hence, another problem arises in that if the timing at which the duty ratio is changed is fixed (constant), it is not possible to surely avoid a large current flowing through the switching element.
The above image forming apparatus achieving a small-sized, hybrid developing device does not take into account the likelihood of a large current flowing through the switching element upon change of the duty ratio and therefore cannot solve the above problems.